The present invention is in the field of glass-ceramics and relates to the production of glass-ceramic materials wherein crystals of beta-spodumene solid solution make up the principal crystal phase.
Glass-ceramic materials are crystalline or semi-crystalline materials produced by the in situ crystallization of glasses. They are typically produced in accordance with a process comprising, first, compounding and melting a batch for a glass which includes nucleating agents to promote glass crystallization; secondly, forming the molten glass into a glass article of the desired configuration; and thirdly, heat treating the glass article in accordance with a time-temperature schedule which promotes the formation of crystal nuclei and the subsequent growth of crystals thereon. Following the crystallization step to completion provides a highly crystalline material wherein the crystals are uniform in size, fine-grained, and uniformly dispersed in a minor residual glassy matrix. The proportion of crystals in a glass-ceramic material is typically quite high, being over 50% and usually over 75% by weight. For a further discussion of glass-ceramic materials and their manufacture, reference may be made to U.S. Pat. No. 2,920,971 to Stookey, and to numerous other texts and patents dealing with this relatively new branch of ceramic technology.
The production of glass-ceramic materials wherein beta-spodumene solid solution comprises the principal crystal phase involves the crystallization of glasses in the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 composition field. Examples of such glasses, and procedures whereby they may be converted into beta-spodumene glass-ceramics, are disclosed in U.S. Pat. No. 2,920,971, as well as in U.S. Pat. Nos. 2,960,801; 2,960,802; 3,148,994; 3,157,522; 3,582,371 and numerous others. Beta-spodumene glass-ceramics have been produced which are white and opaque, and which exhibit good thermal shock resistance by virtue of a very low thermal expansion coefficient.
There are many applications for white, opaque, chemically durable glass-ceramic materials exhibiting low coefficients of thermal expansion. For example Corning Code 9608 glass-ceramic material, a white, opaque beta-spodumene glass-ceramic material comprising about 69.5% SiO.sub.2, 17.6% Al.sub.2 O.sub.3, 2.7% Li.sub.2 O, 2.6% MgO, 1.0% ZnO, 4.7% TiO.sub.2, 0.2% ZrO.sub.2, 0.9% As.sub.2 O.sub.3 and, 0.03% by weight, has been extensively utilized for the manufacture of glass-ceramic cookware. This material exhibits good chemical durability and an average linear coefficient of thermal expansion over the temperature range 25.degree.-300.degree. C. falling in the range of about 10 to about 15.times.10.sup.-7 /.degree. C.
It has been customary in the production of white, opaque glass-ceramics of the beta-spodumene type to utilize crystallization heat treatments comprising peak crystallization temperatures on the order of 1100.degree. C., and crystallization times at peak temperature on the order of 2-4 hours. The heat treatment of lithium aluminosilicate glasses at lower temperatures or for shorter times normally provides transparent to translucent glass-ceramics and/or glass-ceramics containing beta-quartz or beta-eucryptite in substitution for or in addition to the desired beta-spodumene crystal phase.
Methods by which white, opaque beta-spodumene glass-ceramics could be produced from lithium aluminosilicate glasses at lower crystallization temperatures or using shorter crystallization times could have substantial economic value, especially if significant overall energy savings could result therefrom. Such considerations have led to the recent development of spontaneous beta-spodumene glass-ceramics which are provided by the spontaneous crystallization of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 glasses of specified composition as they are formed and cooled from the melt. U.S. Pat. No. 3,985,533 to Grossman, for example, describes one family of beta-spodumene glass-ceramics not requiring a post-forming heat treatment to achieve crystallization. However, spontaneous beta-spodumene glass-ceramics are typically higher in thermal expansion and lower in chemical durability than beta-spodumene glass-ceramics produced by more conventional processes.
It is a principal object of the present invention to provide a white, opaque beta-spodumene glass-ceramic material exhibiting both good chemical durability and low thermal expansion which is produced utilizing a crystallization heat treatment employing lower temperatures and shorter times than have been required for the production of such material in the prior art.
It is a further object of the invention to provide thermally crystallizable glass compositions which can be converted to white, opaque glass-ceramics using such a heat treatment.
It is a further object of the invention to provide an improved process for the production of white, opaque beta-spodumene glass-ceramics exhibiting low thermal expansion and good chemical durability.
Other objects and advantages of the invention will become apparent from the following detailed description thereof.